Power semiconductor devices are used to carry large currents and support high voltages. Power semiconductor devices are typically fabricated from silicon carbide or gallium nitride based semiconductor materials. One widely used power semiconductor device is the Schottky diode.
Power Schottky diodes typically have a vertical structure where the anode contact is formed on a first major surface (e.g., the bottom surface) of a semiconductor layer structure, and the cathode contact is formed on the other major surface (e.g., the top surface). The semiconductor layer structure may or may not include an underlying substrate. Herein, the term “semiconductor layer structure” refers to a structure that includes one or more semiconductor layers such as semiconductor substrates and/or semiconductor epitaxial layers.
A conventional silicon carbide power Schottky diode typically has a silicon carbide substrate having a first conductivity type (e.g., an n-type substrate), on which an epitaxial layer structure having the first conductivity type (e.g., n-type) is formed. This epitaxial layer structure (which may comprise one or more layers) functions as a drift region of the device. The Schottky diode may include an “active region” that is formed on and/or in the drift region and a termination structure that surrounds the active region. The active region acts as a main junction for blocking voltage in the reverse bias direction and providing current flow in the forward bias direction. Typically, a large number of Schottky diodes are formed on a semiconductor wafer. Each Schottky diode may have a “unit cell” structure in which the active region of the device includes a plurality of individual diodes that are disposed in parallel to each other and that together function as a single power Schottky diode. Each power Schottky diode will typically have its own edge termination structure. The edge termination structure may help reduce undesired electric field crowding effects that may occur at the edges of the active region. After the wafer is fully formed and processed, the wafer may be diced to separate the individual edge-terminated power Schottky diodes. The portion of the wafer included in each individually singulated device is called the substrate.
A power Schottky diode is designed to block (in the reverse blocking state) or pass (in the forward operating state) large voltages and/or currents. For example, in the reverse blocking state, a power Schottky diode may be designed to sustain hundreds or thousands of volts of electric potential. However, as the applied reverse voltage approaches or passes the voltage level that the device is designed to block (the reverse breakdown voltage level), non-trivial levels of reverse leakage current may begin to flow through the diode. As the reverse voltage is increased further, the reverse leakage current may increase rapidly, and the diode will enter reverse breakdown and no longer block the reverse voltage. Current leakage can also occur for other reasons, such as electric field crowding at the edges of the active region and/or failure of an edge termination and/or the primary junction of the device. If the reverse voltage on the device is increased past the reverse breakdown voltage to a critical level, the increasing electric field may result in an uncontrollable and undesirable runaway generation of charge carriers within the Schottky diode, leading to a condition known as avalanche breakdown.
For a vertical Schottky diode, the reverse blocking voltage rating is typically determined by the thickness and the doping concentration of the drift region. The reverse blocking voltage rating of the device may be increased by reducing the doping concentration of the drift region and/or by increasing the thickness of the drift region. During the design phase, a desired reverse blocking voltage rating is selected, and then the thickness and doping of the drift region may be chosen based on the desired reverse blocking voltage rating. Since the drift region is the current path for the device in the forward “on” state, the decreased doping concentration and/or increased thickness of the drift region may result in a higher on-state resistance for the device. Thus, there is an inherent tradeoff between the on-state resistance (and hence the forward voltage that will turn the device on) and the reverse blocking voltage.
FIG. 1 is a schematic cross-sectional diagram of a conventional power Junction Barrier Schottky (“JBS”) diode 10. As shown in FIG. 1, the JBS diode 10 includes a cathode contact 20, an ohmic contact layer 22, an n-type substrate 24, an n-type drift region 30, a p-type blocking junction 40, a channel 50 in an upper portion of the n-type drift region, a Schottky contact 42 and an anode contact 44. The cathode contact 20 and the anode contact 44 may each comprise a highly conductive metal layer. The Schottky contact 42 may comprise a layer that forms a Schottky junction with the drift region 30 and may comprise, for example, an aluminum layer. The n-type substrate 24 may comprise a silicon carbide substrate that is heavily doped with n-type impurities such as nitrogen or phosphorous. The ohmic contact layer 22 may comprise a metal that forms an ohmic contact to n-type silicon carbide so as to form an ohmic contact to the silicon carbide substrate 24. The drift region 30 may comprise an epitaxially grown n-type silicon carbide semiconductor region. The p-type blocking junction 40 may be a p-type implanted region in an upper portion of the drift region 30 that is heavily implanted with p-type dopants. The channel 50 may be defined between two adjacent p-type blocking junctions 40 (only one of which is shown in FIG. 1). Current flows through the channel 50 when the diode 10 is in its forward on-state.